Perchlorate (ClO4−) contamination has primarily occurred in association with manufacturing of missiles, fireworks, and other industrial processes and has been recorded in 38 US states. Military applications have also resulted in contaminants such as nitrate and Royal Dutch Explosives (RDX) present with perchlorate as co-contaminants. Perchlorate contamination poses a significant health threat, and toxicological studies have demonstrated that it interferes with iodine uptake into the thyroid gland disrupting thyroid function. Although national standards have yet to be established, the Commonwealth of Massachusetts has set a maximum contaminant limit for perchlorate of 2 μg/L.
Perchlorate is highly soluble and stable in water and hence cannot be removed by conventional drinking water treatment processes such as filtration or air stripping. As an alternative, biological reduction of perchlorate has been investigated by several researchers and is thought to be the most cost-effective process for perchlorate removal. Certain bacteria have shown to metabolize perchlorate to chloride, which is harmless to the environment. A number of organic electron donors have been investigated for perchlorate reduction including acetate, hydrogen, elemental iron, thiosulfate, ethanol, desugared molasses and municipal wastewater, using pure and mixed cultures. Current isolates are characterized as mostly denitrifying, facultative anaerobes which can either degrade or co-metabolize perchlorate.
Excessive amounts of nitrogen discharged from decentralized, sub-surface wastewater treatment systems, or septic systems, degrades natural waters. Conventional septic systems remove at best about 23% of the nitrogen in the influent wastewater. Adding onsite, denitrification treatment, in a comparative evaluation of four previous, conventional technologies, showed maximum nitrogen removal reaching only 66%. Thus, there is a great need for cost-effective technologies applicable to onsite wastewater treatment that can achieve relatively higher percentages of nitrogen removal.
Nitrogen in wastewater is typically in the form of ammonia (NH3) and organic nitrogen. Common aerobic soil bacteria convert ammonia and organic nitrogen to nitrate (NO3−) in soil, through the process of nitrification. A common treatment process is the reduction of NO3− to gaseous nitrogen, N2, gas through biological denitrification.
Biological denitrification is carried out in a bioreactor by bacteria that use nitrate as an energy source under anoxic conditions. Nitrate reduction is coupled with oxidation of an electron donor. Reduction of nitrate to nitrogen gas proceeds as follows:NO3−→NO2−→NO→N2O→N2  (1)Heterotrophic biological denitrification is commonly coupled with nitrification for removing total nitrogen from domestic and industrial wastewater. Heterotrophic denitrifying bacteria require an organic carbon source for energy and cell synthesis. An internal organic carbon source can be provided by recirculating nitrified wastewater to an anoxic zone in the bioreactor; however, total nitrogen removal is limited in these systems. Methanol is often favored as an external electron donor owing to its lower cost and sludge production compared with other organic carbon sources. However, methanol is difficult to handle, deliver and store and residual methanol in the effluent may pose a toxicity problem.
Autotrophic denitrification using sulfur has been studied since the latter half of the last century. A number of common soil bacteria are able to use reduced sulfur compounds as electron donors and respire on nitrate in the absence of oxygen. The process requires no external carbon and produces low amounts of biomass. A stoichiometric equation for autotrophic denitrification using elemental sulfur (S0) as an electron donor is55S0+20CO2+50NO3−+38H2O+4NH4+→4C5H7O2N+55SO42−+25N2+64H+  (2)Based on this equation, for each gram of NO3−—N removed approximately 0.64 g of organic cells and 2.5 g of sulfate (SO42−) are generated.
Benefits of this sulfur-based treatment approach include: denitrification can take place under aerobic conditions, eliminating the need to deoxygenate the influent wastewater; autotrophic bacteria yield less sludge; and these bacteria produce less of the greenhouse gas nitrous oxide (N2O) than do heterotrophic bacteria.
Denitrification using sulfur and biological organisms in a bioreactor requires maintaining an appropriate chemical, nutrient and energetic environment for the biochemical reactions to proceed favorably. One of the important chemical parameters is acidity (pH) in the aqueous medium to which the bacteria are exposed. As can be seen in Eq. 2, above, the products of the denitrifying reactions with elemental sulfur as a reactant include the creation of hydrogen ion [H+] as a product. Increasing concentrations of hydrogen ion correspond to increasing acidity in water, or a lower pH (where pH=−log[H+]). Buffering refers to balancing pH, absorbing the acidity in water, or restoring alkalinity.
Alkalinity relates to a measure of total hydroxyl [OH−], bicarbonate [HCO3−], and carbonate ion [CO3(2−)] available in natural water to balance acidity. For systems in which the carbonate species provide the major source of alkalinity, such as in the wastewater environment present in wastewater treatment processes, Total Alkalinity. (TALK) can be more precisely defined as[TALK]=[HCO3−]+[OH−]+2[CO32−]−[H+]  (3)Total Alkalinity is commonly expressed in milligrams per liter (mg/L) as calcium carbonate (CaCO3). A half mole of CaCO3 (50 grams) is charge-equivalent to one mole of H+ ion (because each dissolved CaCO3 molecule produces a carbonate ion with double negative charge); thus, 50 mg/L as CaCO3 is 1 milli-equivalent per liter (meq/L), i.e., charge-equivalent to one milli-mole of H+ ion per liter.
As the reaction in Eq. 2 drives forward, the reaction products increase the acidity of the aqueous environment of the bioreactor, which in turn can inhibit the ability of the bacteria to drive denitrification. In this reaction, to remove a gram of nitrate, 4.5 gram equivalents alkalinity as CaCO3 are consumed.
Therefore, it is advantageous to introduce a source of alkalinity that can sufficiently buffer the acidity as it builds up. It is additionally desirable that this source of alkalinity provide buffering capacity at a rate that matches the needs of the denitrification system for optimal biochemical and chemical processes.
Sulfur and limestone autotrophic denitrification (SLAD) processes have been known and studied since the 1950s including mixing these materials in a packed-bed bioreactor. Most of these processes have only been studied at the scale of the laboratory bench, however. A number of researchers have used reduced sulfur compounds for biological denitrification of domestic wastewater, industrial wastewater, and drinking water. Several early studies focused on thiosulfate (S2O32−) as an electron donor.
The SLAD approach was further studied in the 1990s to provide optimum design criteria for the SLAD process. This provided a reportedly optimum sulfur dosage and a loading rate in a SLAD system, the minimum average retention time for water in the reactor, and the nitrate loading rate corresponding to the maximum nitrate removal efficiency.
However, despite the traditional SLAD processes being well-studied, in actual practice problems exist that limit using known SLAD processes to clean wastewater at the field scale. One problem is that the SLAD systems have required frequent “backwashing” (or “backflushing”), i.e., running a flow of water counter to the direction of the normal treatment flow, in order to dislodge sludge and regain active biochemistry. Following this backwashing, there is typically a time-lag in regaining denitrification efficiency. A second problem has been that nitrite (NO2−) has increased in the effluent when the hydraulic retention time (HRT) has been less than 6 hours and the nitrogen loading exceeds 200 g/day NO3−—N per cubic meter of the SLAD media.
Autotrophic perchlorate reduction is carried out by organisms that use inorganic compounds, such as hydrogen or reduced iron or sulfur compounds, as electron donors and inorganic carbon as a carbon source. The use of inorganic electron donors eliminates the problem of carry-over of excess organic carbon into the product water. Since these organisms are slow growing, very little excess biomass is produced. Hydrogen has been reported in several studies to be comparable in perchlorate degradation rates to organic electron donors such as acetate.
Perchlorate reducing organisms are ubiquitous in nature; many species of denitrifiers have been shown to be capable of reducing perchlorate using either organic electron donors or hydrogen. However, perchlorate reduction using elemental sulfur has not been reported previously to have been successful; it may require a specialized consortium of microorganisms and/or specialized biochemical conditions in a bioreactor.
Therefore, a need exists for new methods, processes, technology and system designs that can overcome these problems and provide a cost-effective system for reducing perchlorate and/or nitrogen in wastewater.